Get Started with PyTorch

This tutorial walks through the process of converting an existing PyTorch script to use Ray Train.

Learn how to:

1. Configure a model to run distributed and on the correct CPU/GPU device.

2. Configure a dataloader to shard data across the workers and place data on the correct CPU or GPU device.

3. Configure a training function to report metrics and save checkpoints.

4. Configure scaling and CPU or GPU resource requirements for a training job.

5. Launch a distributed training job with a TorchTrainer class.

Quickstart

For reference, the final code is as follows:

from ray.train.torch import TorchTrainer
from ray.train import ScalingConfig

def train_func(config):
    # Your PyTorch training code here.

scaling_config = ScalingConfig(num_workers=2, use_gpu=True)
trainer = TorchTrainer(train_func, scaling_config=scaling_config)
result = trainer.fit()

1. train_func is the Python code that executes on each distributed training worker.

2. ScalingConfig defines the number of distributed training workers and whether to use GPUs.

3. TorchTrainer launches the distributed training job.

Compare a PyTorch training script with and without Ray Train.

PyTorchPyTorch + Ray Train

import tempfile
import torch
from torchvision.models import resnet18
from torchvision.datasets import FashionMNIST
from torchvision.transforms import ToTensor, Normalize, Compose
from torch.utils.data import DataLoader
from torch.optim import Adam
from torch.nn import CrossEntropyLoss

# Model, Loss, Optimizer
model = resnet18(num_classes=10)
model.conv1 = torch.nn.Conv2d(1, 64, kernel_size=(7, 7), stride=(2, 2), padding=(3, 3), bias=False)
criterion = CrossEntropyLoss()
optimizer = Adam(model.parameters(), lr=0.001)

# Data
transform = Compose([ToTensor(), Normalize((0.5,), (0.5,))])
train_data = FashionMNIST(root='./data', train=True, download=True, transform=transform)
train_loader = DataLoader(train_data, batch_size=128, shuffle=True)

# Training
for epoch in range(10):
    for images, labels in train_loader:
        outputs = model(images)
        loss = criterion(outputs, labels)
        optimizer.zero_grad()
        loss.backward()
        optimizer.step()

    checkpoint_dir = tempfile.gettempdir()
    checkpoint_path = checkpoint_dir + "/model.checkpoint"
    torch.save(model.state_dict(), checkpoint_path)

Set up a training function

First, update your training code to support distributed training. Begin by wrapping your code in a training function:

def train_func(config):
    # Your PyTorch training code here.

Each distributed training worker executes this function.

Set up a model

Use the ray.train.torch.prepare_model() utility function to:

1. Move your model to the correct device.

2. Wrap it in DistributedDataParallel.

-from torch.nn.parallel import DistributedDataParallel
+import ray.train.torch

 def train_func(config):

     ...

     # Create model.
     model = ...

     # Set up distributed training and device placement.
-    device_id = ... # Your logic to get the right device.
-    model = model.to(device_id or "cpu")
-    model = DistributedDataParallel(model, device_ids=[device_id])
+    model = ray.train.torch.prepare_model(model)

     ...

Set up a dataset

Use the ray.train.torch.prepare_data_loader() utility function, which:

1. Adds a DistributedSampler to your DataLoader.

2. Moves the batches to the right device.

Note that this step isn’t necessary if you’re passing in Ray Data to your Trainer. See Data Loading and Preprocessing.

 from torch.utils.data import DataLoader
-from torch.utils.data import DistributedSampler
+import ray.train.torch

 def train_func(config):

     ...

     dataset = ...

     data_loader = DataLoader(dataset, batch_size=worker_batch_size)
-    data_loader = DataLoader(dataset, batch_size=worker_batch_size, sampler=DistributedSampler(dataset))
+    data_loader = ray.train.torch.prepare_data_loader(data_loader)

     for X, y in data_loader:
-        X = X.to_device(device)
-        y = y.to_device(device)

     ...

Tip

Keep in mind that DataLoader takes in a batch_size which is the batch size for each worker. The global batch size can be calculated from the worker batch size (and vice-versa) with the following equation:

global_batch_size = worker_batch_size * ray.train.get_context().get_world_size()

Report checkpoints and metrics

To monitor progress, you can report intermediate metrics and checkpoints using the ray.train.report() utility function.

+import ray.train
+from ray.train import Checkpoint

 def train_func(config):

     ...
     torch.save(model.state_dict(), f"{checkpoint_dir}/model.pth"))
+    metrics = {"loss": loss.item()} # Training/validation metrics.
+    checkpoint = Checkpoint.from_directory(checkpoint_dir) # Build a Ray Train checkpoint from a directory
+    ray.train.report(metrics=metrics, checkpoint=checkpoint)

     ...

For more details, see Monitoring and Logging Metrics and Saving and Loading Checkpoints.

Configure scale and GPUs

Outside of your training function, create a ScalingConfig object to configure:

1. num_workers - The number of distributed training worker processes.

2. use_gpu - Whether each worker should use a GPU (or CPU).

from ray.train import ScalingConfig
scaling_config = ScalingConfig(num_workers=2, use_gpu=True)

For more details, see Configuring Scale and GPUs.

Launch a training job

Tying this all together, you can now launch a distributed training job with a TorchTrainer.

from ray.train.torch import TorchTrainer

trainer = TorchTrainer(train_func, scaling_config=scaling_config)
result = trainer.fit()

Access training results

After training completes, a Result object is returned which contains information about the training run, including the metrics and checkpoints reported during training.

result.metrics     # The metrics reported during training.
result.checkpoint  # The latest checkpoint reported during training.
result.path     # The path where logs are stored.
result.error       # The exception that was raised, if training failed.

Next steps

After you have converted your PyTorch training script to use Ray Train:

• See User Guides to learn more about how to perform specific tasks.

• Browse the Examples for end-to-end examples of how to use Ray Train.

• Dive into the API Reference for more details on the classes and methods used in this tutorial.